fix: dispatch realtime events on captured asyncio loop

[rupurt]

Summary

• capture the asyncio event loop before SignalR connection setup starts
• route realtime and batched market callbacks through the captured loop with run_coroutine_threadsafe
• drop late SignalR events quietly when the loop is gone instead of trying to create tasks on SignalR threads
• add regression coverage for active-loop, worker-thread, and missing-loop dispatch paths

Why

SignalR callbacks can run on non-asyncio worker threads. The previous fallback attempted to create asyncio tasks from those threads when the stored loop was missing or closed, which produced repeated No event loop available for market_depth event and quote_update event errors.

Validation

• uv run pytest tests/realtime/test_event_handling.py -q
• uv run ruff check src/project_x_py/realtime/event_handling.py src/project_x_py/realtime/connection_management.py tests/realtime/test_event_handling.py